1. Field of the Invention
The present invention relates to a circuit and method used to calibrate and compensate for laser performance in systems such as an optical communications links, medical diagnostic systems and any other system utilizing lasers. Performance compensation is achieved in a non-invasive manner without disruption of the laser signal transmission or other operating parameters of the laser.
2. Description of the Related Art
Market trends demand increased levels of reliability and intelligence in laser systems.
Particularly, in laser signal transmission there is the need to send information with reliable optical power signals. Reliability requires the transmitted signals to maintain a given signal strength as well as other performance parameters. Lasers undergo degradation due to aging, temperature changes, and other effects. This degradation causes the signal strength to be reduced resulting in a decrease of signal-to-noise ratio, extinction ratio and an increased Bit Error Rate.
Prior art has utilized either analog controllers or mixed analog/digital controllers as opposed to the Digital Controller (111) shown in FIG. 1 below in the detailed description.
Challenges with the Measurement Process.
In order to properly control the laser Module (106), the Digital Controller (111) requires feedback information from light output (107). When the control system is operational, obtaining feedback information becomes problematic since the light output (107) constantly changes depending on the Drive Signal (100) the system is transmitting. Thus any attempts to measure the light output (107) will encounter errors, which can render the feedback information unusable.
To perform a measurement of the light output (107), the Drive Signal (100) needs to be maintained at a fixed power level in order for the system to produce a steady value of the Light Output (107) so that calibration adjustments can be made. This procedure disrupts the signal transmission and, because of this, the transmitter cannot send information over the optical communications channel while the calibration is carried out. Disruption in communication is contrary to the goals of high reliability and 100% up time in present systems.
If the Photodiode Sensor (109) is slow relative to the Laser Module (106), once the system is transmitting information, the Photodiode Sensor (109) cannot be effectively utilized to calibrate the amplitude of the Light Output (107) because the sensor may have a slower response than the laser. The Photodiode Sensor (109) operates as a band-limiting filter converting the response to a variety of waveforms as follows:
An exponential rise and decay of the Photodiode Sensor (109) output is produced for a serial stream of the Drive Signal (100) comprised of all ones. For this data sequence, the average of the Photodiode Sensor (109) will exhibit the highest value;
A Photodiode Sensor (109) output with an average value close to zero volts will be obtained for a serial stream of the Drive Signal (100) comprised of all zeros;
The output of the Photodiode Sensor (109) will exhibit an average voltage value, which will between the maximum and minimum values described above depending on a generic sequence of date with mixed values of ones and zeros.
To carry out a power measurement of the light output (107), the prior art has utilized a variety of methods. In one method the process has been as follows.
The digital input Drive Signal (100) is disconnected and a peak value of analog current from the Modulation Current Generator (103) is applied to the laser;
The Light Output (107) is measured with an optical power meter.
The Photodiode Sensor (109) generates a corresponding signal proportional to the light output;
Adjustments are made in the Controller (111) in order to increase the magnitude of the optical power coming out of the laser to the desired level;
The adjustments in the Controller (111) affect the Bias Current Generator (102) and Modulation Current Generator (103), which in turn affect the Light Output (107) of the Laser Module (106);
This approach has the disadvantage of requiring disconnection of the laser control system (114). Disconnection in many systems, such as communications equipment, is not acceptable.
The process for another possible solution previously used is as follows:
Produce a circuit to synthesize a high frequency calibration signal;
Inject the calibration signal into the node between the Modulation Current Generator (103) and the laser module (106);
Sense the calibration signal with the Photodiode Sensor (109);
Add a special filter circuit between the photodiode sensor (109) and the Digital Controller (111);
Detect the magnitude of the calibration signal with the Digital Controller (111).
The problem with this solution is that it affects the information transmitted. This prior art solution has a significant impact on the reliability of information transmission because it essentially inserts noise into the transmitted signal. Furthermore this approach increases complexity and cost due to an additional calibration signal generator, a calibration signal injection circuit, plus filter and detection circuits.
Because of errors in power measurement, transmission systems in prior art generally overdrive the laser to account for variations of temperature, aging and other effects. This approach significantly reduces the life of the laser.